1. The molecular mechanisms involved in DNA replication are being studied. Attempts are being made to reproduce in vitro, the in vivo pathways of replication of bacteriophage lambda DNA in extracts of lambda infected bacteria and of viral SV40 DNA in extracts of viral infected or transformed animal cells. The present emphasis in both cases is to gain insight into the process of initiation of chromosome replication. 2. The molecular mechanisms involved in DNA recombination are also being studied. The reaction by which prophage lambda is excised from the bacterial chromosome has been reconstituted with purified proteins in vitro. The interactions of these proteins with each other and DNA are being characterized.